The present invention relates generally to the mounting of an electronic component on a heat spreader, and in particular to the mounting of an electronic component on a heat spreader with a connection of a terminal of the component to a printed circuit board.
In the prior art, electronic components, such as RF (radio frequency) power transistors, are soldered to a circuit board and to a heat spreader. Typically, at least one of the leads of the RF power transistor is soldered to a pad on an edge of the circuit board and the base of the RF power transistor is soldered to a surface of the heat spreader. This requires that the soldering take place on multiple planes, that is the base of the transistor must be soldered to a heat spreader while the leads of the transistor are soldered to the top of the printed circuit board. It is desirable that this process be automated in order to reduce the cost of manufacturing. This soldering process is further complicated, however, by the fact that available RF power transistor packages have a seating plane height which is not always equal to the desired printed circuit board thickness. As a result, prior art methods of solder screening are not usable because a simple solder paste screening cannot be applied to multiple height levels in a single screening. Thus, hand application of solder paste is the only known method of applying solder paste to the base of the RF power transistor in the prior art.
Prior art solder preforms have been used for multi-plane soldering operations wherein the solder is applied to the base of the transistor for reflow. However, this is a hand operation because of the very weak mechanical rigidity of the preform, and thus is a poor candidate for automation.
FIGS. 1 and 2 depict prior art solutions for adjusting the height of the RF power transistor 100 so that the leads 102 of the RF power transistor 100 align with the surface 104 of the printed circuit board 106. To accomplish this, either a pedestal 108 is machined on the heat spreader 110 such that the bottom 112 of the RF power transistor 100 can be soldered to the pedestal 108 as shown in FIG. 1, or as shown in FIG. 2, the printed circuit board 106 is notched at surface 104 in order to adjust the height of the RF power transistor 100. These processes in the prior art provide a solution to the height differential at a very high dollar cost. This is because it requires a secondary machine procedure. Although it is known to add a metal shim to provide the pedestal function as depicted in FIG. 1, it is a hand operation that is required to dispense the solder onto the shim. This manual operation often results in solder shorts and yield loss. For higher power transistors and similar components it is necessary to use a heat spreader in order to dissipate the heat generated during operation of the RF power transistor. Heat spreaders are known that are composed of metallic materials, such as copper or aluminum, or are composed of non-metallic materials such as graphite. If necessary, as is also known in the prior art, a well is formed in the heat spreader 110 as opposed to the pedestal 108.
Consequently, a need exists for a structure which allows the component to be adjusted in height with regards to the spacing between the surface of the printed circuit board and the surface of the heat spreader. The prior art solutions all include a non-automated step which significantly adds to the cost of manufacturing. Furthermore, in the market for components, such as power transistors, there are very few suppliers and obtaining different size packages is not feasible. Therefore, there is a need for a solution which does not involve a custom package design for the component.